Efficient uplink transmission of channel state information

ABSTRACT

A UE in a wireless communication network transmits succinct, direct channel state information to the network, enabling coordinated multipoint calculations such as joint processing, without substantially increasing uplink overhead. The UE receives and processes reference symbols over a set of non-uniformly spaced sub-carriers, selected according to a scheme synchronized to the network. The frequency response for each selected sub-carrier is estimated conventionally, and the results quantized and transmitted to the network on an uplink control channel. The non-uniform sub-carrier selection may be synchronized to the network in a variety of ways.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 61/172,484, filed Apr. 24, 2009, titled “Channel StateInformation Feedback by Digital Loopback,” and is a Continuation of U.S.Regular application Ser. No. 12/555,966 filed Sep. 9, 2009, their entirecontents are respectively incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to wireless communicationsystems, and in particular, the present invention relates to anefficient system and method of providing channel state information fromuser equipment to a wireless communication network.

BACKGROUND

Wireless communication networks transmit communication signals in thedownlink over radio frequency channels from fixed transceivers, known asbase stations, to mobile user equipment (UE) within a geographic area,or cell. The UE transmit signals in the uplink to one or more basestations. In both cases, the received signal may be characterized as thetransmitted signal, altered by channel effects, plus noise andinterference. To recover the transmitted signal from a received signal,a receiver thus requires both an estimate of the channel, and anestimate of the noise/interference. The characterization of a channel isknown as channel state information (CSI). One known way to estimate achannel is to periodically transmit known reference symbols, also knownas pilot symbols. Since the reference symbols are known by the receiver,any deviation in the received symbols from the reference symbols (onceestimated noise/interference is removed) is caused by channel effects.An accurate estimate of CSI allows a receiver to more accurately recovertransmitted signals from received signals. In addition, by transmittingCSI from the receiver to a transmitter, the transmitter may select thetransmission characteristics—such as coding, modulation, and thelike—best suited for the current channel state. This is known aschannel-dependent link adaptation.

Modern wireless communication networks are interference limited. Thenetworks typically process transmissions directed to each UE in a cellindependently. Transmissions to other UEs in the same cell are regardedas interference at a given UE—giving rise to the term inter-cellinterference. One approach to mitigating inter-cell interference isCoordinated Multipoint (CoMP) transmission. CoMP systems employ numeroustechniques to mitigate inter-cell interference, including MIMO channels,numerous distributed antennas, beamforming, and Joint Processing.

Joint Processing (JP) is a CoMP transmission technique currently beingstudied for Long Term Evolution (LTE) Advanced. In JP, transmissions tomultiple UEs are considered jointly, and a global optimization algorithmis applied to minimize inter-cell interference. That is, JP algorithmsattempt to direct transmission energy toward targeted UEs, whileavoiding the generation of interference at other UEs. To operateeffectively, JP systems require information about the transmissionchannels. There are two ways in which the channel information, or CSI,is fed back to system transmitters: Precoding Matrix Indicator (PMI) andquantized channel feedback.

PMI feedback, specified in LTE Release 8, is essentially arecommendation of a transmission format by each UE. A plurality ofpre-defined precoding matrices are designed offline and known at boththe base station and UE. The precoding matrices define various sets ofdownlink coding and transmission parameters. Each UE measures itschannel, and searches through the precoding matrices, selecting one thatoptimizes some quantifiable metric. The selected precoding matrix is fedback or reported to the base station. The base station then considersall recommended precoding matrices, and selects the precoding andtransmission parameters that implement a globally optimal solution overthe cell. In the scenarios contemplated when Release-8 LTE was designed,PMI feedback works well, due to a high correlation betweenrecommendations from UEs and the actual desirable transmissionparameters. PMI feedback compression reduces uplink bandwidth byexploiting the fact that only part of the channel—the “strongdirections,” i.e., the signal space—needs to be fed back to thetransmitter.

In JP CoMP applications, it is unlikely that the desired transmissionformat (which achieves interference suppression) will coincide with atransmission format recommended by a UE. No recommending UE has anyknowledge about other UEs that will be interfered by the transmission tothe recommending UE. Additionally, the recommending UE has no knowledgeof transmissions scheduled to other UEs that will interfere with itssignals. Also, PMI feedback compression reduces bandwidth by reportingonly the part of the channel of interest to transmissions directed tothe recommending UE. While this increases uplink efficiency fornon-cooperative transmission, it is disadvantageous for cooperativetransmission, as it denies the network information about the channelthat may be useful in the JP optimization.

In quantized channel feedback, UEs attempt to describe the actualchannel. In contrast to PMI feedback, this entails feeding backinformation about not only the signal space but also the complementaryspace (the “weaker space,” also somewhat inaccurately referred to as the“null space”) of the channel. Feedback of the whole channel results inseveral advantages. With full CSI available at the network, coherent JPschemes can suppress interference. Additionally, the network can obtainindividualized channel feedback by transmitting unique reference symbolsto each UE. This enables flexible and future-proof implementations of avariety of JP transmission methods, since the methods are essentiallytransparent to the UE.

Even without JP CoMP transmission, CSI at the network can solve one ofthe most fundamental problems plaguing current wireless system—theinaccuracy in channel-dependent link adaptation due to the network notbeing able to predict the interference experienced by the UEs (a problemclosely related to the well-known flash-light effect). Once the networkknows the CSI of bases near each UE, the network can accurately predictthe SINR at each UE resulting in significantly more accurate linkadaptation.

Even though the advantages of direct CSI over PMI feedback are clear,the major issue with direct CSI feedback is bandwidth. Full CSI feedbackrequires a high bitrate to transmit the CSI from each UE to the network.Time-frequency uplink channel resources must be used to carry the CSIfeedback on the uplink channel, making these resources unavailable fortransmitting user data on the uplink—the CSI feedback transmissions arethus pure overhead, directly reducing the efficiency of uplink datatransmissions. Conveying direct CSI feedback to the network withoutconsuming excessive uplink resources stands as a major challenge ofmodern communication system design.

SUMMARY

According to one or more embodiments described and claimed herein, a UEin a wireless communication network transmits succinct, direct channelstate information to the network, enabling coordinated multipointcalculations such as joint processing, without substantially increasinguplink overhead. The UE receives and processes reference symbols over aset of non-uniformly spaced sub-carriers, selected according to a schemesynchronized to the network. The frequency response for each selectedsub-carrier is estimated conventionally, and the results quantized andtransmitted to the network on an uplink control channel. The non-uniformsub-carrier selection may be synchronized to the network in a variety ofways.

One embodiment relates to a method of reporting channel stateinformation by a UE operative in a wireless communication network inwhich downlink data is modulated onto a plurality of sub-carriers, eachhaving a different frequency. A plurality of known reference symbols arereceived over a subset of the plurality of sub-carriers. A set ofnon-uniformly spaced sub-carriers is selected using a selection schemesynchronized to the network. A frequency response is estimated for eachselected sub-carrier. The frequency responses are quantized andtransmitted to the network via an uplink control channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a wireless communicationnetwork.

FIG. 2A is a time-frequency plot showing reference symbol transmissionfrom a single antenna port.

FIG. 2B is a time-frequency plot showing reference symbol transmissionfrom two antenna ports.

FIG. 2C is a time-frequency plot showing reference symbol transmissionfrom three antenna ports.

FIG. 3 is a flow diagram of a method of reporting CSI feedback by a UE.

FIG. 4A is a graph of the in-phase component of a representative channelresponse, depicting the quantized channel estimates reported to thenetwork.

FIG. 4B is a graph of the quadrature component of a representativechannel response, depicting the quantized channel estimates reported tothe network.

DETAILED DESCRIPTION

For the purpose of clear disclosure and full enablement, the presentinvention is described herein as embodied in a wireless communicationnetwork based on Orthogonal Frequency Division Multiplex (OFDM)modulation. More specifically, embodiments herein are based on theEvolved Universal Terrestrial Radio Access (E-UTRA) system, which isalso commonly referred to as the Long-Term Evolution (LTE) of the widelydeployed WCDMA systems. Those of skill in the art will readilyappreciate that these systems are representative only and not limiting,and will be able to apply the principles and techniques of the presentinvention to a wide variety of wireless communication systems, baseddifferent access and modulation methods, given the teachings of thepresent disclosure.

FIG. 1 depicts a wireless communication network 10. The network 10includes a Core Network (CN) 12, communicatively connected to one ormore other networks 14, such as the Public Switched Telephone Network(PSTN), the Internet, or the like. Communicatively connected to the CN12 are one or more Radio Network Controllers (RNC) 16, which in turncontrol one or more Node B stations 18. The Node B 18, also known as abase station, includes radio frequency (RF) equipment and antennasnecessary to effect wireless radio communications with one or more userequipment (UE) 20 within a geographic region, or cell 22. As depicted,the Node B 18 transmits data and control signals to the UE 20 on one ormore downlink channels, and the UE similarly transmits data and controlsignals to the Node B 18 on the uplink.

Interspersed within the data on the downlink transmission, the network10 transmits reference symbols, also known in the art as pilot symbols,to assist the UEs 20 performing channel estimation on the downlinkchannel responses. FIG. 2A depicts example of the reference symbolresources for the LTE network 10 of FIG. 1, when the Node B 18 transmitson a single antenna port. The depicted grid plots sub-carriers on theordinate axis (frequency increasing downwardly) and time (increasing tothe right) on the abscissa axis. Note that the time periods areorganized into frames, with even-numbered and odd-numbered slotsdepicted. Each grid element is an OFDM time-frequency resource element,which may carry a data symbol, reference symbol, or neither. FIGS. 2Band 2C depict reference symbol transmissions when the Node B 18transmits on two and four antenna ports, respectively.

The reference symbols enable the UE to employ a wide range of standardtechniques to estimate the frequency responses of all sub-carriers.Since the values of the reference symbols are known to the UE 20, theestimation quality is generally highest on the sub-carriers occupied bythe reference symbols.

FIG. 3 depicts a method of reporting CSI by a UE 20 to the network 10,according to one embodiment. The UE 20 receives known reference symbolsover some of the sub-carriers transmitted to it, as depicted in FIG. 2(block 102). The UE 20 selects a set of non-uniformly spacedsub-carriers, on which to perform channel estimation for CSI feedback(block 104). In one embodiment, selection of sub-carriers is limited tothose on which reference symbols are transmitted, because the channelestimation quality is generally highest at these sub-carriers. However,in other embodiments, the UE 20 additionally selects one or moresub-carriers that do not include reference symbols. As discussed furtherherein, the selection of non-uniformly spaced sub-carriers is performedaccording to a scheme that is synchronized in some manner with thenetwork. The UE 20 estimates the frequency response of the channel(block 106). The frequency response samples associated with the selectedsub-carriers are then quantized or encoded by a suitable source encoderinto digital bits (block 108). The digital bits are then transmitted viaa suitable control channel from the UE 20 to the network 10 (block 110).The control channel provides suitable error detection and correctioncoding as well as radio resources (transmission power and frequencyresource allocation) to ensure proper reception quality at the network10. The method then repeats.

FIGS. 4A and 4B depict the representative channel frequency response forin-phase (FIG. 4A) and quadrature (FIG. 4B) components of a receivedsignal. Out of 50 reference symbol sub-carriers tones over a 5 MHz band,a subset of 15 non-uniformly spaced reference symbol sub-carriers isselected. These samples are depicted as stars in FIGS. 4A and 4B. Thesamples do not always lie on the frequency response curves due primarilyto two sources of noise. First, it is assumed the mean square error(MSE) of the UE channel estimator at the reference symbol sub-carriersis ˜20 dB. Secondly, the I and Q parts of the selected channel estimatesare digitized independently by a simple 4-bit uniform quantizer. Theresulting average quantization noise is about ˜22 dB. With this setup, atotal of 15*4*2=120 bits are fed back by the UE 20.

With uniform sampling, the Nyquist theorem dictates that samples(sub-carriers) must be selected at twice the highest frequency of thechannel frequency response curve, to fully characterize the curve. Usingnon-uniform samples, however, far fewer than the Nyquist criterion ofsub-carriers may be selected, with a high probability of accuratereconstruction of the channel frequency response curve by the network10. Accordingly, by selecting non-uniformly spaced sub-carriers, the UE20 may fully characterize the channel and provide direct CSI feedback,without imposing excessive overhead on the uplink channel.

Upon receipt by the network 10, the received CSI feedback bits aredemodulated and inverse quantized. The complete frequency domain channelcoefficients may be estimated by setting a time-domain tap-delay channelmodel based on the received sub-carrier samples. Applying, e.g., a FastFourier Transform (FFT) to the estimated delay coefficients yields afrequency-domain response very close to that depicted in FIGS. 4A and4B. Detail of the network-side processing of CSI feedback based onchannel estimates of non-uniformly spaced sub-carriers is disclosed incopending U.S. patent application Ser. No. 12/555,973, assigned to theassignee of the present application, filed concurrently herewith, andincorporated herein by reference in its entirety. The network-sideprocessing assumes the network 10 is aware of which non-uniformlyselected sub-carriers were analyzed by the UE 20. Thus, the UE 20 mustselect the non-uniformly spaced sub-carriers according to a scheme,protocol, or formula that is synchronized with the network 10. There arenumerous ways to accomplish this.

In one embodiment, the set of non-uniformly spaced sub-carriers ischanged for each batch, or iteration, of CSI feedback reporting, in amanner coordinated with the network 10.

In one embodiment, the set of non-uniformly spaced sub-carriers isselected based on pseudo-randomized indices with synchronized readingoffset. For example, the pseudo-randomized indices may be obtained bytaking sequentially the indices produced by a pseudo-random numbergenerator. The pseudo-random number generator may be computed based onan algebraic modification of the input reading indices. For example, thealgebraic modification may be based quadratic permutation polynomials(QPP), as described in the 3GPP Technical Specifications 36.212,“Multiplexing and channel coding,” incorporated herein by reference. Asanother example, the algebraic modification may be based on afinite-field computation.

As another example of pseudo-randomized indices with synchronizedreading offset, the pseudo-randomized indices may be obtained by takingsequentially the indices produced by an interleaver. The interleaver maybe computed based on a column-interleaved rectangular array, asdescribed in section 5.1.4.2.1 of the 3GPP Technical Specifications36.212.

As yet another example, the sequential reading of indices may besynchronized between UE 20 and the network 10 via an agreed indexreading offset. The agreed index reading offset may be obtained in manyways. It may be transmitted explicitly in the same transport channelwith the digital bits from the UE 20 to the network 10. Alternatively,the agreed index reading offset may be implicitly computed based on a UE20 identification number, a sub-frame number, a CSI feedback batch oriteration count, an antenna identification number, a network-sideidentification number, or the uplink control channel recourse index(e.g., where it is the index of the first resource block for the uplinkcontrol channel). The agreed index reading offset may be implicitlycomputed based on the downlink control channel resource index (e.g.,where it is the index of the first resource block for the downlinkcontrol channel). Alternatively, the agreed index reading offset may betransmitted from the network 10 to the UE 20 prior to the UE 20performing channel estimation, or may be pre-agreed between the network10 and the UE 20. In either case, the index reading offsets may bestored in the UE 20 as a loop-up table.

In one example, the set of non-uniformly spaced sub-carriers is selectedby initially selecting uniformly spaced sub-carriers, and then applyingpseudo-randomized dithering, with a key synchronized to the network 10,to the uniformly spaced sub-carriers to generate the set ofnon-uniformly spaced sub-carriers. In one embodiment, the maximum spanof the pseudo-randomized dithering is selected to be smaller than theuniform spacing in the uniformly spaced indices. The generation ofpseudo-randomized dithering may be computed based on algebraicmodification of an input key. As described above with respect to thenon-uniform sub-carrier selection, the pseudo-randomized dithering maybe obtained by taking sequentially the indices produced by aninterleaver or a pseudo-random number generator, with the generation ofindices computed by the UE 20 based on any of the factors above, orcommunicated between the network 10 and UE 20, as also described above.

A more general formulation of the selection of sub-carriers, channelestimation, and quantization and reporting of CSI feedback is nowpresented. The frequency response of a channel at frequency f and time tcan be expressed in terms of the time domain channel taps h(l;t) havingdelays τ_(i) as follows:

${H\left( {f;t} \right)} = {\sum\limits_{l = 0}^{L - 1}{{h\left( {l;t} \right)}^{{- {j2\pi}}\; f\; \tau_{l}}}}$

At each reporting iteration or time t, the following steps are performedby the UE 20:

First, the UE 20 forms an estimate of the downlink channel at a numberof sub-carriers. As described above, known reference signals aretransmitted from each network antenna (see FIG. 2A-2C), and the UE 20can use these reference signals to form an estimate of the channel at anumber of sub-carriers using standard techniques. These estimates aredenoted by the following N×1 vector:

g(t)=[ÎI(f ₁ ;t)ÎI(f _(2;) ;t)Λ ÎI(f _(N) ;t)]^(T)

Where ÎI(f;t) is the UE-estimated frequency response of the channel atfrequency f and time t.

Second, for each reporting instant, the UE 20 forms a number of linearcombinations of elements of g(t), i.e., the UE 20 multiplies the vectorg(t) by a mixing matrix P(t), of size M×N, to get a new vector r(t) ofsize M×1, according to:

r(t)=P(t)×g(t).

In embodiments where the elements of P(t) comprise only the values zeroor one, P(t) “selects” elements from the vector of channel estimates ofnon-uniform sub-carriers g(t) according to each row of P(t). In someembodiments, the results of computations or communications describedabove to select the reading offsets of pseudo-randomized indices may bestored in the mixing matrix P(t). In more general embodiments, however,the elements of P(t) are not restricted to the values zero or one. Forexample, the elements may comprise fractional values between zero andone, in which case they act as weighting factors as well as selectors.Additionally, the elements may comprise complex values.

The mixing matrix P(t) may be changed for different sets of iterationsof CSI feedback. In one embodiment, the selection of P(t) may be by around-robin selection from among a collection of mixing matrices. In oneembodiment, the changing of P(t) may comprise selecting different rowcompositions. For example, the selection of different row compositionsmay be based on the round-robin use of a plurality of rows. As anotherexample, it may be based on a pseudo-randomized selection from aplurality of rows. The pseudo-randomized selection of rows may beobtained by taking sequentially the indices produced by an interleaveror pseudo-random number generator, where the indices may be communicatedor computed in any manner described above.

In one embodiment, the mixing matrix P(t) comprises rows having at mostone non-zero element. In another embodiment, the mixing matrix P(t)comprises rows given by an orthonormal matrix, such as a Hadamardmatrix. In yet another embodiment, the mixing matrix P(t) comprises rowsgiven by a unitary matrix. In still another embodiment, the mixingmatrix P(t) may be generated by first generating pseudo-random matrices{A_(i)} with independent Gaussian-distributed entries, performing a QRdecomposition on each A_(i), and using each resulting unitary Q matrixas a candidate for P(t).

However the mixing matrix P(t) is derived, after the multiplication withg(t), the elements of the product matrix r(t) are quantized using aquantizer Q_(r)(•) to obtain a number of bits, denoted as the vectorb(t), representing the vector r(t). The bits in b(t) are thentransmitted to the network 10 using an uplink control channel. As knownin the art, the transmission process may include adding redundancy suchas CRC, FEC, and the like, to ensure reliable transmission to thenetwork 10.

In the embodiments described above, the UE 20 determines the parametersfor selecting non-sequential sub-carriers and/or the ditheringparameters to generate a non-sequential selection of sub-carriers, suchas the selection of indices for a pseudo-random number generator,autonomously or quasi-autonomously from the network 10 (although, ofcourse, whatever selection mechanism is employed must be synchronizedwith the network 10). In some embodiments, however, the network 10directly controls these and other parameters via transmissions to the UE20 in the downlink.

In one embodiment, the network 10 determines the set of sub-carriers(f₁, . . . , f_(N)), for which the UE 20 should estimate channelresponse and place in vector g(t). In one embodiment, the network 10determines the mixing matrix P(t) that the UE 20 should use at eachreporting instance. In one embodiment, the network 10 determines thequantizer Q_(r)(•) that the UE 20 uses at each reporting instance, whichdetermines, for example, how many bits are used to quantize each elementof r(t). In one embodiment, the network 10 determines how often the CSIfeedback reports should be transmitted by the UE 20 on the uplink. Inall these embodiments, the network 10 communicates the relevantdeterminations to the UE 20 in downlink communications. Additionally, ofcourse, the network 10 schedules the time-frequency uplink resources onwhich the CSI feedback reports shall be transmitted by the UE 20, justas for any uplink communications.

In a typical network 10, each UE 20 might have to report CSI feedback onmultiple downlink channels, from multiple different Node Bs 18. Sincethe path loss between each UE 20 and Node B 18 is different, thedownlink channels to be estimated and reported by each UE 20 will havedifferent average power. With a fixed bitrate budget for CSI feedbackallocated to each UE 20, a problem arises as how this total fixedbitrate should be divided among the different downlink channels seen bythe UE 20.

If a channel between a given UE 20 and a given Node B 18 is extremelyweak, the signals transmitted from the Node B 18 will have very littleimpact at the receiver of the UE 20. Hence, there is little need for theUE 20 to report CSI feedback corresponding to the channels that are veryweakly received at the UE 20. Accordingly, in one embodiment, the UE 20allocates a larger portion of its allocated CSI feedback bitrate to thedownlink channels that are relatively strong, than to the channels thatare relatively week. Given a set of average channel signal strengthsg(1), g(2), . . . , g(B), and a total CSI feedback allocation of K bits,the network 10 can allocate its total bitrate budget among the variouschannels. In one embodiment, the network 10 performs the allocationaccording to a Generalized Breiman, Friedman, Olshen, and Stone (BFOS)Algorithm, as described by E. A. Riskin in the paper, “Optimal BitAllocation via Generalized BFOS Algorithm,” published in the IEEE Trans.Info. Theory, 1991, the disclosure of which is incorporated herein byreference in its entirety.

In one embodiment, the reporting of CSI feedback may be spread out overa plurality of iterations of CSI feedback. That is, a set ofnon-uniformly spaced subcarriers are selected, and a frequency responseis calculated for each subcarrier. The frequency responses arequantized. However, rather than transmit all of the quantized frequencyresponse data to the network at once, the reporting is spread over twoor more iterations of CSI feedback. For example, at time N, some number,e.g. ten, subcarriers are selected, and their frequency responsescalculated and quantized (possibly jointly). The quantized bits may thenbe transmitted to the network over the next ten time intervals, e.g., attimes N+1, N+2, . . . , N+10. Of course, reports for two subcarrierscould be transmitted at a time, using five CSI reporting intervals, orany other permutation. This reporting method minimizes the uplinkbandwidth required for reporting CSI captured at one time.

In another embodiment, a persistent form of CSI reporting comprisesselecting one or more subcarrier and calculating its frequency response.The quantized frequency response is then transmitted to the network.Over time, the selection of subcarriers is non-uniform. For example, afirst subcarrier is selected at time N, and its quantized frequencyresponse is transmitted to the network at reporting interval N+1. Atthat time, a new subcarrier (at a different frequency) is selected, andits quantized frequency response is transmitted to the network at thereporting interval N+2. Similarly, two or more subcarriers may beselected during any given CSI generation and reporting interval. Thisreporting method minimizes the uplink bandwidth by spreading both thesubcarrier selection, and the reporting of quantized CSI data, overtime.

Embodiments described herein significantly reduce CSI feedbackbandwidth, while enabling highly accurate CSI availability to thenetwork. This efficiently allows for the implementation of advancednetwork protocols such as joint processing in coordinated multipointtransmission, without consuming excess uplink transmission resources.

The present invention may, of course, be carried out in other ways thanthose specifically set forth herein without departing from essentialcharacteristics of the invention. The present embodiments are to beconsidered in all respects as illustrative and not restrictive, and allchanges coming within the meaning and equivalency range of the appendedclaims are intended to be embraced therein.

1. A method of reporting channel state information (CSI) by userequipment (UE) operative in a wireless communication network in whichdownlink data is modulated onto a plurality of sub-carriers, each havinga different frequency, comprising, at each iteration: receiving aplurality of known reference symbols over a subset of the plurality ofsub-carriers; selecting a set of non-uniformly spaced sub-carriers usinga selection scheme synchronized to the network; estimating a frequencyresponse for each selected sub-carrier; quantizing the frequencyresponses; and transmitting the quantized frequency responses to thenetwork via an uplink control channel.
 2. The method of claim 1 whereinthe selected sub-carriers all include one or more reference symbols. 3.The method of claim 1 wherein one or more of the selected sub-carriersdoes not include any reference symbols.
 4. The method of claim 1 furthercomprising applying Forward Error Correction coding to the quantizedfrequency responses prior to transmitting them.
 5. The method of claim 1wherein selecting a set of non-uniformly spaced sub-carriers comprisesselecting a different set of non-uniformly spaced sub-carriers for eachCSI reporting iteration.
 6. The method of claim 1 wherein selecting aset of non-uniformly spaced sub-carriers using a selection schemesynchronized to the network comprises selecting the set based onpseudo-randomized indices with an index reading offset synchronized tothe network.
 7. The method of claim 6 wherein the pseudo-randomizedindices comprise the sequential indices produced by one of aninterleaver and a pseudo-random number generator.
 8. The method of claim7 wherein the interleaver or pseudo-random number generator is computedby algebraic modification of input reading indices.
 9. The method ofclaim 8 wherein the algebraic modification is based on quadraticpermutation polynomials.
 10. The method of claim 8 wherein the algebraicmodification is based on finite-field computation.
 11. The method ofclaim 7 wherein the interleaver is computed based on acolumn-interleaved rectangular array.
 12. The method of claim 11 whereinthe column-interleaved rectangular array is derived according to section5.1.4.2.1 of the 3GPP Technical Specification 36.212, “Multiplexing andchannel coding.”
 13. The method of claim 7 wherein the sequentialindices are synchronized between the UE and the network by apredetermined index offset.
 14. The method of claim 13 furthercomprising transmitting the predetermined index offset to the networkwith the quantized frequency responses.
 15. The method of claim 13further comprising computing the predetermined index offset based on aunique UE identification number.
 16. The method of claim 13 furthercomprising computing the predetermined index offset based on a sub-framenumber.
 17. The method of claim 13 further comprising computing thepredetermined index offset based on an iteration count of CSI feedback.18. The method of claim 13 further comprising computing thepredetermined index offset based on a unique antenna identificationnumber.
 19. The method of claim 13 further comprising computing thepredetermined index offset based on a unique network identificationnumber.
 20. The method of claim 13 further comprising computing thepredetermined index offset based on a resource index of the uplinkcontrol channel.
 21. The method of claim 20 wherein the uplink controlchannel resource index is an index of a first resource block for theuplink control channel.
 22. The method of claim 13 further comprisingcomputing the predetermined index offset based on a resource index of adownlink control channel.
 23. The method of claim 22 wherein thedownlink control channel resource index is an index of a first resourceblock for the downlink control channel.
 24. The method of claim 13further comprising receiving the predetermined index offset from thenetwork.
 25. The method of claim 13 wherein the predetermined indexoffset is programmed into the UE.
 26. The method of claim 25 furthercomprising retrieving the predetermined index offset from a look-uptable stored in memory on the UE.
 27. The method of claim 1 whereintransmitting the quantized frequency responses to the network via anuplink control channel comprises transmitting less than the entire setof quantized frequency responses in one CSI iteration.
 28. The method ofclaim 27, further comprising transmitting one or more quantizedfrequency responses in a following CSI iteration.
 29. The method ofclaim 1 wherein selecting a set of non-uniformly spaced sub-carriersusing a selection scheme synchronized to the network comprises selectingone or more, but less than the entire set, of sub-carriers in each CSIiteration, such that the set of sub-carriers selected over two or moreCSI iterations is non-uniform.
 30. The method of claim 1 whereinselecting a set of non-uniformly spaced sub-carriers using a selectionscheme synchronized to the network comprises: selecting a set ofuniformly spaced sub-carriers; and applying pseudo-random dithering,generated using a key synchronized to the network, to the set ofuniformly spaced sub-carriers to generate the set of non-uniformlyspaced sub-carriers.
 31. The method of claim 30 wherein the maximum spanof the pseudo-random dithering is smaller than the uniform spacing inthe set of uniformly spaced sub-carriers.
 32. The method of claim 30further comprising computing the pseudo-random dithering by an algebraicmodification of an input key.
 33. The method of claim 30 wherein thepseudo-random dithering is obtained from the sequential indices producedby one of an interleaver and a pseudo-random number generator.
 34. Themethod of claim 33 wherein the interleaver or pseudo-random numbergenerator is computed by algebraic modification of input readingindices.
 35. The method of claim 34 wherein the algebraic modificationis based on quadratic permutation polynomials.
 36. The method of claim34 wherein the algebraic modification is based on finite-fieldcomputation.
 37. The method of claim 33 wherein the interleaver iscomputed based on a column-interleaved rectangular array.
 38. The methodof claim 37 wherein the column-interleaved rectangular array is derivedaccording to section 5.1.4.2.1 of the 3GPP Technical Specification36.212, “Multiplexing and channel coding.”
 39. The method of claim 33wherein the sequential indices are synchronized between the UE and thenetwork by a predetermined index offset.
 40. The method of claim 39further comprising transmitting the predetermined index offset to thenetwork with the quantized frequency responses.
 41. The method of claim39 further comprising computing the predetermined index offset based ona unique UE identification number.
 42. The method of claim 39 furthercomprising computing the predetermined index offset based on a sub-framenumber.
 43. The method of claim 39 further comprising computing thepredetermined index offset based on an iteration count of CSI feedback.44. The method of claim 39 further comprising computing thepredetermined index offset based on a unique antenna identificationnumber.
 45. The method of claim 39 further comprising computing thepredetermined index offset based on a unique network identificationnumber.
 46. The method of claim 39 further comprising computing thepredetermined index offset based on a resource index of the uplinkcontrol channel.
 47. The method of claim 46 wherein the uplink controlchannel resource index is an index of a first resource block for theuplink control channel.
 48. The method of claim 39 further comprisingcomputing the predetermined index offset based on a resource index of adownlink control channel.
 49. The method of claim 46 wherein thedownlink control channel resource index is an index of a first resourceblock for the downlink control channel.
 50. The method of claim 39further comprising receiving the predetermined index offset from thenetwork.
 51. The method of claim 39 wherein the predetermined indexoffset is programmed into the UE.
 52. The method of claim 51 furthercomprising retrieving the predetermined index offset from a look-uptable stored in memory on the UE.
 53. The method of claim 1 whereinselecting a set of non-uniformly spaced sub-carriers comprises receivingfrom the network an indication of the sub-carriers to be selected. 54.The method of claim 1 wherein quantizing the frequency responsescomprises: receiving from the network an indication of a quantizer touse; and quantizing the frequency responses using the indicatedquantizer.
 55. The method of claim 54 wherein the indication of aquantizer to use specifies the number of bits used to quantize eachfrequency response.
 56. The method of claim 1 wherein transmitting thequantized frequency responses to the network via an uplink controlchannel comprises: receiving from the network an indication how often totransmit quantized frequency responses to the network; and transmittingthe quantized frequency responses to the network at the indicated rate.57. A method of reporting channel state information (CSI) by userequipment (UE) operative in a wireless communication network in whichdownlink data is modulated onto a first plurality of sub-carriers, eachhaving a different frequency, comprising, at each iteration: receiving aplurality of known reference symbols over a subset of the firstplurality of sub-carriers; estimating a frequency response for each of asecond plurality of sub-carriers; collecting the frequency responsesinto a vector; selecting a group of frequency responses by multiplyingthe vector of frequency responses by a mixing matrix that issynchronized to the network, to yield a vector of selected frequencyresponses; quantizing the selected frequency responses; and transmittingthe quantized frequency responses to the network via an uplink controlchannel.
 58. The method of claim 57 wherein estimating a frequencyresponse for each of a second plurality of sub-carriers comprisesestimating a whitened channel response for each of the second pluralityof sub-carriers.
 59. The method of claim 57 wherein the mixing matrix ischanged for each iteration of CSI reporting.
 60. The method of claim 59wherein, for each iteration, a mixing matrix is selected in round-robinorder from a collection of predetermined mixing matrices.
 61. The methodof claim 59 wherein, for each iteration, a different row composition isselected for the mixing matrix.
 62. The method of claim 61 wherein therow composition is selected in round-robin order from a collection ofpredetermined rows.
 63. The method of claim 61 wherein the rowcomposition is selected by a pseudo-random selection from a collectionof predetermined rows.
 64. The method of claim 63 wherein thepseudo-random row selection is obtained from the sequential indicesproduced by one of an interleaver and a pseudo-random number generator.65. The method of claim 64 wherein the interleaver or pseudo-randomnumber generator is computed by algebraic modification of input readingindices.
 66. The method of claim 65 wherein the algebraic modificationis based on quadratic permutation polynomials.
 67. The method of claim65 wherein the algebraic modification is based on finite-fieldcomputation.
 68. The method of claim 64 wherein the interleaver iscomputed based on a column-interleaved rectangular array.
 69. The methodof claim 68 wherein the column-interleaved rectangular array is derivedaccording to section 5.1.4.2.1 of the 3GPP Technical Specification36.212, “Multiplexing and channel coding.”
 70. The method of claim 64wherein the sequential indices are synchronized between the UE and thenetwork by a predetermined index offset.
 71. The method of claim 70further comprising transmitting the predetermined index offset to thenetwork with the quantized frequency responses.
 72. The method of claim70 further comprising computing the predetermined index offset based ona unique UE identification number.
 73. The method of claim 70 furthercomprising computing the predetermined index offset based on a sub-framenumber.
 74. The method of claim 70 further comprising computing thepredetermined index offset based on an iteration count of CSI feedback.75. The method of claim 70 further comprising computing thepredetermined index offset based on a unique antenna identificationnumber.
 76. The method of claim 70 further comprising computing thepredetermined index offset based on a unique network identificationnumber.
 77. The method of claim 70 further comprising computing thepredetermined index offset based on a resource index of the uplinkcontrol channel.
 78. The method of claim 77 wherein the uplink controlchannel resource index is an index of a first resource block for theuplink control channel.
 79. The method of claim 70 further comprisingcomputing the predetermined index offset based on a resource index of adownlink control channel.
 80. The method of claim 79 wherein thedownlink control channel resource index is an index of a first resourceblock for the downlink control channel.
 81. The method of claim 70further comprising receiving the predetermined index offset from thenetwork.
 82. The method of claim 70 wherein the predetermined indexoffset is programmed into the UE.
 83. The method of claim 82 furthercomprising retrieving the predetermined index offset from a look-uptable stored in memory on the UE.
 84. The method of claim 57 whereineach row of the mixing matrix has at most one nonzero element.
 85. Themethod of claim 57 wherein the mixing matrix is an orthonormal matrix.86. The method of claim 85 wherein the orthonormal matrix is a Hadamardmatrix.
 87. The method of claim 57 wherein the mixing matrix is aunitary matrix.
 88. The method of claim 87 wherein the mixing matrix isformed by: generating a plurality of pseudo-random matrices withindependent Gaussian-distributed entries; performing a QR decompositionon each random matrix to generate a unitary matrix; and selecting aunitary matrix as the mixing matrix.
 89. The method of claim 57 whereinselecting a group of frequency responses by multiplying the vector offrequency responses by a mixing matrix that is synchronized to thenetwork comprises: receiving from the network an indication of themixing matrix to use, and multiplying the vector of frequency responsesby the indicated mixing matrix.
 90. A User Equipment (UE) operative in awireless communication network in which downlink data is modulated ontoa plurality of sub-carriers, each having a different frequency,comprising: one or more antennas; selection means for selecting a set ofsub-carriers using a selection scheme synchronized to the network; afrequency estimator operative to estimate a frequency response forselected sub-carriers; a quantizer operative to quantize selectedfrequency responses; and a transmitter operative to transmit selectedquantized frequency responses to the network via an uplink controlchannel.